1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst. More specifically, this invention relates to feed and catalyst contacting.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to generally by those skilled in the art as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, for transporting catalyst between reaction and regeneration zones, and for combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
Despite the long existence of the FCC process, techniques are continually sought for improving product recovery both in terms of product quantity and composition, i.e. yield and selectivity. Two facets of the FCC process that have received attention are recovery of adsorbed products from the spent FCC catalyst and initial contacting of the FCC feed with the regenerated catalyst. Improvement in the recovery of hydrocarbons from spent catalyst directly improves yields while better initial feed and catalyst contacting tends to benefit yield and selectivity.
One direct way of recovering additional products is by more complete stripping of hydrocarbons from the spent catalyst. More complete stripping removes hydrocarbons from the catalyst that are lost by combustion when the spent catalyst enters the regeneration zone. Common methods to more completely strip catalyst raise the temperature of the spent catalyst in the stripping zone as a means of desorbing hydrocarbons from spent catalyst prior to regeneration. One system for heating spent catalyst in the stripping zone employs indirect heat transfer. A more common method of heating spent catalyst in the stripping zone mixes higher temperature regenerated catalyst with the spent catalyst in the stripping zone. U.S. Pat Nos. 3,821,103 and 2,451,619 describe systems for direct heating of spent catalyst with hot regenerated catalyst.
A variety of devices and piping arrangements have been employed to initially contact catalyst with feed. U.S. Pat. No. 5,017,343 is representative of devices that attempt to improve feed and catalyst contacting by maximizing feed dispersion. Another approach to improved feed and catalyst contacting is to increase the penetration of the feed into a flowing stream of catalyst. U.S. Pat. No. 4,960,503 exemplifies this approach where a plurality of nozzles surround an FCC riser to shoot feed into a moving catalyst stream from a multiplicity of discharge points. While these methods do improve distribution of the feed into the hot regenerated catalyst stream, there is still a transitory period of poor distribution when the relatively small quantities of the hydrocarbon feed disproportionately contact large quantities of hot catalyst. This poor thermal distribution results in non-selective cracking and the production of low value products such as dry gas.
The processing of increasingly heavier feeds and the tendency of such feeds to elevate coke production and yield undesirable products has led to new methods of contacting FCC feeds with catalyst. Recently methods of contacting FCC catalyst for very short contact periods have been of particular interest. U.S. Pat. No. 4,985,136 the contents of which are hereby incorporated by reference discloses an ultra short contact time for the fluidized catalytic cracking process that contacts an FCC feed with a falling curtain of catalyst for a contact time of less than 1 second followed by a quick separation. U.S. Pat. No. 5,296,131 the contents of which are hereby incorporated by reference discloses a similar ultra short contact time process that uses an alternate falling catalyst curtain and separation arrangement. The ultra short contact time system improves selectivity to gasoline while decreasing coke and dry gas production by using high activity catalyst that contacts the feed for a relatively short period of time. The inventions are specifically directed to zeolite catalysts having high activity. The short contact time arrangements permit the use of much higher zeolite content catalysts that increase the usual 25-30% zeolite contents of the FCC catalyst to amounts as high as 40-60% zeolite in the cracking catalyst. These references teach that shorter hydrocarbon and catalyst contact time is compensated for by higher catalyst activity. One drawback of most short contact time systems is the continued cracking of hydrocarbons that remain adsorbed or entrained with the catalyst after the initial stage of short contacting. These entrained and adsorbed hydrocarbons remain in a dense bed for long periods of time that can promote overcracking.
Another method that has been used to vary the contacting of feed with catalyst is the blending of spent and regenerated catalyst. U.S. Pat. No. 5,346413 issued to Lomas describes a method of blending spent catalyst with regenerated to increase the total catalyst to oil ratio and reduce the temperature of the catalyst blend. U.S. Pat. No. 5,462,652 issued to Wegerer uses a mixture of spent and regenerated catalyst to contact catalyst in an ultra short contact time arrangement. However, the hydrocarbons that remain on the catalyst again experience long residence time in a dense catalyst bed that still creates the problem of over cracking.
An additional limitation of ultra-short contacting system is their limited flexibility in feed contacting. Ultra-short contacting provides one type of contacting that may not be the optimal cracking regime for all feeds. Present arrangements for ultra short feed contacting cannot be altered to provide longer contact times when such contacting would be desirable for certain feeds.
Improved methods are sought for disbursing feed within the catalyst stream while avoiding localized overheating of the feed and for achieving thermal equilibrium between the relatively hotter catalyst and the relatively cooler feed. Such methods would reduce the localized overheating of the feed or the severity of the feed heating caused by the large temperature differentials between the feed and the catalyst which both contribute to feed over cracking while also avoiding or reducing the overcracking of entrained and adsorbed hydrocarbons. It would also be desirable to have an apparatus and process that is readily adaptable to providing ultra short feed contacting and longer transport conduit cracking.
It is an object of this invention to improve the control of cracking reaction time for light readily cracked hydrocarbons and more refractory heavy hydrocarbons.
Another object of this invention is to provide initial ultra short contacting of the feedstream followed by transport conduit cracking of adsorbed or entrained hydrocarbons.
It is a further object of this invention that to alternately or simultaneously provide ultra short catalyst contacting of product and traditional upflow or downflow contacting of hydrocarbon and feed in a catalyst transport conduit.